Rather than thinking about the slow boil, the sensible conclusion for a non-US person is not to invest in the US. Why risk it? Why risk another change to the risks?

Fair point, and this new tax policy could definitely be changed for the worse again in the future.

But if the US stock market's returns continued to outpace other countries, as they've done for quite some time (past performance not predictive... you know the line), it could still be a net advantage to invest in the US even if a person hypothetically was subjected to the maximum 20% additional tax.

I haven't confirmed these numbers, but here's what AI says were the European stock market performances vs. S&P500 in the last 25 years:

S&P 500 (USA): ~7.5% CAGR
MSCI Europe Index: ~5.5% CAGR
Germany (DAX): ~6.0% CAGR
France (CAC 40): ~4.8% CAGR
UK (FTSE 100): ~4.0% CAGR
Other European Countries: ~3-5% (Limited data for smaller markets (e.g., Spain, Italy, Netherlands)

This doesn't consider valuation levels and may be less relevant for a person who's picking individual stocks rather than indexes, but if these were the next 25 years of returns, and if a non-US investor was subjected to the current worst-case of a 20% additional tax, that would reduce the 7.5% CAGR on the S&P to 6.0% CAGR, which still beats all other European countries (tie with Germany)

I think a key point too is the dynamic nature of US politics. A completely different administration will be in office in over 3.5 more years. A key question in my mind is how likely is the current anti-globalization mentality is likely to continue, especially if the US starts feeling the effects of even a slight decoupling from global markets.